chore: move bigframes package to separate repo

[tswast]

https://github.com/google/bigframes is the new location for bigframes.

See internal issue b/517198791
🦕

[snippet-bot]

Here is the summary of changes.

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• packages/bigframes/samples/polars/create_polars_df_with_to_arrow_test.py:17, tag bigquery_dataframes_to_polars

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[gemini-code-assist]

Code Review

This pull request completely removes the bigframes package from this repository, including all source code, tests, configuration files, Kokoro CI/CD configurations, and release manifests. References to bigframes in the root README.rst have been updated to point to its new standalone repository on GitHub. As there are no review comments provided, I have no feedback to offer.

[tswast]

Doctest is the BigFrames doctest. I'll send a corresponding internal change removing these BigFrames-related Kokoro jobs.

Edit: mailed cl/964068930

[parthea]

LGTM but please also allow @JoeWang1127 to review